class CreatedMixin(object):
    @declared_attr
    def created_by(cls):
        return Column(Integer, ForeignKey('user.user_id',
                  onupdate="cascade", ondelete="restrict"))

    @declared_attr
    def updated_by(cls):
        return Column(Integer, ForeignKey('user.user_id',
                      onupdate="cascade", ondelete="restrict"))

    created_at = Column(DateTime, nullable=False, default=dt.now())
    updated_at = Column(DateTime, nullable=False, default=dt.now(),
                        onupdate=dt.now())

class LogMixin(CreatedMixin, DeclarativeBase):
    __tablename__ = 'ttt'

event.listen(LogMixin, 'before_insert', created_by)
event.listen(LogMixin, 'before_update', updated_by)

class Net(LogMixin):
    """Net or subnet
    """
    __tablename__ = 'net'

    cidr = Column(postgresql.CIDR, index = True, unique = True)
    description = Column(UnicodeText())